Observing the renewable energy transition from a European perspective

District Heating with Seasonal Storage in Vojens Denmark

In Vojens, in the not too sunny Denmark, the largest storage of seasonal solar heat project in the world is operational since 2016. Denmark is unique in that it has many district heating systems in place, that can be fairly easily adapted to cooperate with large arrays of solar thermal collectors and seasonal storage of the resulting heat in large pits, filled with gravel and water.

Operator: Vojens Fjernvarme
Storage volume: 200,000 m3
Depth storage: 13 m
Circumference: 610 m
Solar collector area: 70,000 m2
Pump capacity: 50 m3/hour
Charging time: 5 months
Location: old gravel pit
Insulation: 60 cm clay and plastic sheet
Number of households: 2,000
Solar heat coverage connected households: 45%
Consequences heating bill: 10-15% less (for 45% no more fuel, just amortization)
CO2-emissions avoided: 6,000 Tonnes/year
Other energy sources: 3xgas engines, 10 MW electric boiler, absorption heat pump, gas boilers
Price pit storage in 2020: 30 €/m3 for storage volume > 100,000 m3
Price storage pit per household: 1,500 €
Installation overhead per household: 100 m3 storage volume and 35 m2 solar thermal collectors

The positive news is that if you can get the thermal solar generation and storage scheme financed, your heating bill actually decreases with 10-15%. This could set in motion an avalanche of projects all over Europe, certainly in countries with more solar radiation than Denmark, that is almost everybody.

In the context of the renewable energy transition, focus is on wind and pv-solar. This is not justified. For an average Dutch household of 2.2 persons, the yearly consumption of energy for heating is 34 GJ. For electricity that is merely 12 GJ. The difference is almost a factor of 3.

[Google Maps] – Vojens district heating

[] – Vojens district heating
[] – Seasonal pit heat storages – Guidelines for materials & construction
[] – South-Jutland stores the sun’s heat in the world’s largest pit heat storage
[] – Seasonal thermal energy storage

Similar nearby seasonal storage in Gram.

Gram total area 130,000 m3. A large area indeed, but the financial income from that area from solar heat is guaranteed to outshine (pun intended) other agro-industrial possibilities, not to mention the avoidance of environmental cost.

In Aalborg, they have solar collectors for preheating and CSP to feed the seasonal storage with water of 98 °C

[] – 6.8 MW Solar District Heating System in Taars, Denmark

[] – Seasonal pit heat storage: Cost benchmark of 30 EUR/m³

Another district heating project in Silkeborg.

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